1. Field of the Invention
The present invention relates to a sound insulation/absorption structure, a sound insulation/absorption device, and a structure having these applied thereto and a member constituting the same, which insulate sound by elastic repulsion or absorb the sound by an elastic loss.
2. Description of the Prior Art
The sound insulation performance of a single layer wall improves in proportion to the increasing amount of mass. Thus, a material with large mass, such as a concrete wall, a block wall, a bonded brick wall, lead, and a steel plate, is used to insulate a sound. A sound transmission loss is used as an index to show the sound insulation performance of a wall. The sound transmission loss TL of the single layer wall in the case where the sound is vertically incident on the wall surface is expressed by the following formula (1):
                    TL        =                  10          ⁢                                          ⁢                                    log              10                        ⁡                          [                                                                    (                                                                  r                                                  2                          ⁢                                                      ρ                            0                                                    ⁢                                                      c                            0                                                                                              +                      1                                        )                                    2                                +                                                      (                                                                                            ω                          ⁢                                                                                                          ⁢                          m                                                -                                                  Y                          /                          ω                                                                                            2                        ⁢                                                  ρ                          0                                                ⁢                                                  c                          0                                                                                      )                                    2                                            ]                                                          (        1        )            where ω is an angular frequency, ρ0 is the density of air, c0 is the sound velocity of air, r is the viscous resistance of the wall in the thickness direction, m is the mass of the wall, and y is the elastic constant of the wall in the thickness direction.
FIG. 16 shows the sound transmission loss TL obtained by the formula (1) relative the thickness direction shown in the following formula (2):
                              f          r                =                              1                          2              ⁢                                                          ⁢              π                                ⁢                                    Y              m                                                          (        2        )            
The sound transmission loss TL is proportional to the frequency in 6 dB/oct on the higher frequency side than the resonance frequency fr. This area results from a term including the mass of the formula (1) and is referred to as a mass law.
On the other hand, the sound transmission loss TL is inversely proportional to the frequency in −6 dB/oct on the lower frequency side than the resonance frequency fr. This area results from a term including an elastic constant of the formula (1) and is generally referred to as stiffness control.
In a conventional technique, the resonance frequency fr is provided in a low frequency area. Since the sound insulation performance of a sound insulation wall in an audible area depends on the mass law, the sound insulation performance of the wall deteriorates in proportion to low frequency sound. The sound insulation performance can be improved by increasing the thickness (a surface density), but the increase of the sound transmission loss is 6 dB at most even by doubling the thickness. It is also said that a film or plate with a small surface density hardly ever has the sound insulation performance. On the other hand, a sound of a lower frequency than the resonance frequency fr can be insulated in theory by the action of the wall elasticity.
Thus, problems are pointed out in the conventional sound insulation method whereby the sound insulation performance deteriorates in proportion to low frequency sound and there is a limit to the necessary steps which can be taken to improve the sound insulation especially in collective housing or transport facilities because the sound insulation performance depends in collective housing or transport facilities because the sound insulation performance depends on the surface density.
Since the sound insulation method using stiffness control does not depend on the mass, it is not only possible to take proper sound insulation steps at the places where sound insulation steps could not be taken in the past, but also sound insulation for the low frequency sound can be expected. However, a sound insulation/absorption structure using stiffness control has not been in practical use as yet.
As a sound insulation/absorption structure for bringing stiffness control into view, a sound insulation structure and a sound insulation/absorption complex structure are known, which comprise a frame body, surface materials provided on both sides of the frame body, and a sound absorption material filled within these surface materials, wherein each surface material is formed to have a curved surface shape to increase the stiffness (rigidity) so that the stiffness area in the transmission loss frequency characteristics reaches a frequency higher than the resonance transmission frequency determined by the surface density of the surface material and the spacing of the surface materials (e.g., refer to Japanese Patent Application Publication No. 5-94195).
Further, a sound insulation structure is known, which comprises a frame body, surface materials provided on both sides of the frame body, and a sound absorption material filled between these surface materials, wherein the surface materials are curved to increase the stiffness (rigidity) by pressurizing or depressurizing a space surrounded by the frame body and the surface materials. Sound insulation loss (deficiency) by the resonance transmission is prevented by controlling the vibrations of the surface materials (e.g., refer to Japanese Patent Application Publication No. 6-161463).
A variable sound absorption device is also known, which comprises a piezoelectric material having piezoelectric properties of which the outer periphery is secured, a pair of electrodes provided on both opposite faces of this piezoelectric material, and a negative capacitance circuit adapted to connect between these electrodes, wherein the piezoelectric material is in a curved flat state and the electric properties of the negative capacitance circuit is constituted to be variable, thereby changing an elastic constant and a loss factor of the piezoelectric material (e.g., refer to Japanese Patent Application Publication No. 11-161284).
However, the inventions disclosed in Japanese Patent Application Publication Nos. 5-94195 and 6-161463 refer to a technique to control deformation from a surface friction, in other words, a sound transmission caused by a bending resonance of a sound insulation wall as a result of increasing stiffness, a so-called coincidence, wherein the resonance frequency of this bending is due to the surface friction seen in a mass control domain in addition to the resonance frequency fr in the thickness direction as described above. Accordingly, to attain sound insulation by stiffness control, it is necessary to discuss the resonance frequency fr, that is, the surface density and the elasticity of the in-plane stretching. However, these inventions do not deal with the resonance frequency fr and thus, our problems can not be solved.
Further, the invention disclosed in Japanese Patent Application Publication No. 11-161284 describes in theory that if the film is curved, the attenuation of sound can be increased. However, this invention does not describe that the sound insulation by elastic repulsion (stiffness control) of the film can be attained in less than the resonance frequency f r and the sound insulation performance depends on the mass of the film, the length of the periphery, the elastic constant, and the tensile force. The invention does not describe a sound insulation/absorption structure taking these into consideration. Thus, our problems cannot be solved.